Galvanized steel sheets having a zinc coating layer on the surface are inexpensive and highly anticorrosive steel sheets widely used in the fields of automobiles, home appliances, building materials and the like.
In addition to the enhancement in performance, the weight reduction is recently promoted in steel sheets used in the above fields, and this trend has led to a demand for steel sheets having higher strength. As a result, the use of high-strength and anticorrosive galvanized steel sheets has been on the rise.
In a general continuous galvanizing line, the galvanizing treatment is preceded by pretreatment steps in which the surface of a steel sheet is reduced and activated by annealing treatment in a H2—N2 reducing gas atmosphere and thereafter the steel sheet is cooled to a temperature suited for galvanizing while avoiding contact with air. Finally, the steel sheet is immersed in the galvanizing bath to produce a galvanized steel sheet.
To increase the strength of a steel sheet, solid solution strengthening elements such as silicon and manganese are added. When a high-strength steel sheet as a base steel sheet containing large amounts of these elements is subjected to annealing and galvanizing treatments, silicon and manganese that are prone to oxidation are concentrated on the surface of the steel sheet and form oxides in a reducing atmosphere generally used in annealing treatment. Such oxides decrease the wettability of the steel sheet surface with respect to the molten zinc in the subsequent galvanizing treatment, thereby causing bare-spot defects.
When the atmosphere during from annealing treatment until galvanizing treatment has a high H2 concentration, hydrogen absorbed in the steel sheet during annealing treatment is liberated after the galvanizing treatment and accumulated between the base steel sheet and the zinc coating layer. The accumulated hydrogen is released out of the coating layer during alloying treatment of the galvanized steel sheet to produce a Zn—Fe alloyed galvanized steel sheet. Without the alloying treatment, the galvanized steel sheet suffers blistering defects in which the coating layer is raised by the pressure of the accumulated hydrogen.
To address the problems described above, Japanese Unexamined Patent Application Publication No. 2010-255111 discloses a method in which the oxygen potential is lowered by controlling the dew point of the furnace atmosphere during annealing treatment to not more than −45° C. and thereby galvanizing treatment is performed in the absence of silicon and manganese oxides. Further, Japanese Unexamined Patent Application Publication No. 2010-255113 discloses a method in which the hydrogen concentration in the furnace atmosphere during annealing treatment is controlled to not less than 25% to increase the ability of hydrogen to reduce the steel sheet, thus preventing the occurrence of bare-spot defects on the steel sheet.
To remedy blistering due to the absorption of hydrogen, for example, Japanese Unexamined Patent Application Publication No. 5-295483 discloses a method in which a small amount of titanium is added to the steel sheet to precipitate titanium compounds such as TiC and TiN, and hydrogen absorbed in the steel from the atmosphere is caused to remain trapped in voids formed around the precipitates, thereby suppressing the liberation of hydrogen after the galvanizing treatment.
While the method of Japanese Unexamined Patent Application Publication No. 2010-255111 may suppress bare-spot defects irrespective of the Si and Mn contents in steel, technical difficulties are encountered in maintaining the dew point in the annealing furnace at a low level, giving rise to a need of capital investments in new facilities. In the method of Japanese Unexamined Patent Application Publication No. 2010-255113, increasing the hydrogen concentration during annealing treatment causes the steel sheet to absorb an excessively large amount of hydrogen, resulting in the occurrence of blistering. Further, the method of Japanese Unexamined Patent Application Publication No. 5-295483 entails the limitation of steel composition to control blistering and the operation of continuous galvanizing line is adversely affected.
It could therefore be helpful to provide a high-strength galvanized steel sheet based on a Si- and Mn-containing steel sheet and having an excellent surface appearance without bare-spot defects or blistering, and provide a method of manufacturing such a steel sheet.